Pacemaker and method of operating same that provides functional atrial cardiac pacing with ventricular support

ABSTRACT

A special type of AV/PV hysteresis is provided in a dual-chamber pacemaker. A long AV delay is initially provided, thereby affording as much opportunity as possible for natural AV conduction to occur. Such long AV delay is automatically shortened should AV block occur. Periodic scanning for the return of AV conduction (absence of AV block) is performed so that the AV delay can be returned to its long value as soon as possible. In one embodiment, the pacemaker &#34;learns&#34; the natural conduction time (AR interval) of the patient and thereafter uses such learned natural conduction time as a reference against which subsequently measured AR intervals are compared to better distinguish conducted ventricular contractions from ectopic, pathologic, or other nonconducted ventricular contractions (e.g., PVC&#39;s). If the measured AR interval is approximately the same as the &#34;learned&#34; AR interval, then the R-wave at the conclusion of the measured AR interval is presumed to be a conducted R-wave that signals the return of AV conduction, and the AV delay is lengthened back to its original value. If, on the other hand, the measured AR interval is significantly different than the &#34;learned&#34; natural conduction time, then the R-wave at the conclusion of the measured AR interval is presumed to be a nonconducted R-wave, and the AV delay is kept short. In other embodiments, other techniques are used to distinguish a conducted R-wave from a nonconducted R-wave.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/440,599, filed May 15, 1995 now U.S. Pat. No. 5,741,308; which is acontinuation-in-part of application Ser. No. 08/225,226, filed Apr. 8,1994, now abandoned; which is a continuation-in-part of application Ser.No. 08/219,065, filed Mar. 29, 1994, now abandoned; which is acontinuation-in-part of Ser. No. 07/976,153, filed Nov. 13, 1992 U.S.Pat. No. 5,334,220, issued Aug. 2, 94; which applications and patent areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to implantable medical devices andmethods, and more particularly, to an implantable dual-chamber pacemakerthat provides AAI pacing with back-up ventricular support to improvecardiac hemodynamics. Even more particularly, the invention provides adual-chamber pacemaker, and method of operating such pacemaker, whereina type of positive atrioventricular (AV) hysteresis is employed thatprovides a long AV delay, or AV interval (AVI), thereby affording asmuch opportunity as possible for natural cardiac conduction to occur,but where the AVI is automatically shortened during AV block. Further,the invention periodically scans or searches for a return of AVconduction (absence of AV block) so that the AVI can be returned to itslonger value as soon as possible. To assist with determining when AVconduction has returned, the invention further provides severaldifferent ways to differentiate between those ventriculardepolarizations (R-waves) that evidence the return of AV conduction fromthose that do not.

In the above-identified patent and applications, of which thisapplication is a continuation-in-part, there is disclosed an implantabledual-chamber pacemaker that automatically sets its AVI to a value thatis a prescribed amount less than or greater than a measured naturalconduction time of a patient within whom the pacemaker is implanted. Inthe present application, the AVI is set to a long or short valuedepending upon whether AV conduction is present or not. In determiningwhether AV conduction is present or not, the implantable pacemakercircuits monitor those ventricular contractions that normally signal theend of the natural conduction time to determine if such contractionstruly evidence a return of AV conduction or not. One way this is done,for example, is to monitor and "learn" the natural conduction time ofthe patient's heart, and thereafter use such learned natural conductiontime to better distinguish naturally, conducted ventricular contractions(which signal the return of AV conduction, and hence signal that the AVIcan be lengthened) from ectopic or pathologic ventricular contractions(which do not signal the return of AV conduction, and hence whichsuggest that the AVI should be kept short).

BACKGROUND OF THE INVENTION

For a thorough background description of the physiology of a humanheart, as well as a description of the basic operation of an implantablepacemaker, reference should be made to the various patents and patentapplications cited herein. That which is presented below is a briefsummary of such background information.

As is known in the art, the basic function of the heart is to pump(circulate) blood throughout the body thereby delivering oxygen andnutrients to the various tissues and removing waste products and carbondioxide therefrom. The heart is divided into four chambers comprised oftwo atria and two ventricles. The atria are the collecting chambersholding the blood that returns to the heart until the ventricles areready to receive this blood. The ventricles are the primary pumpingchambers. The pumping function of the heart is achieved by a coordinatedcontraction of the muscular walls of the atria and the ventricles.

As is also known in the art, the atria are more than simple collectingchambers. The atria contain the heart's own spontaneous pacemaker, thesinus node, that controls the rate at which the heartbeats or contracts.Furthermore, atrial contraction helps to fill the ventricles,contributing to optimal filling of the ventricles, thus maximizing theamount of blood that the heart is able to pump with each contraction,i.e., maximizing the hemodynamic efficiency of the heart. In the normalheart, an atrial contraction is followed, after a short period of time(normally 120 to 200 ms), by a ventricular contraction, i.e., aconducted R-wave.

The period of cardiac contraction during which the heart actively ejectsthe blood into the arterial blood vessels is called systole. The periodof cardiac relaxation during which the chambers are being filled withblood is called diastole. Atrial and ventricular systole are sequencedallowing the atrial contraction to help optimally fill the ventricle.This sequencing is termed AV synchrony.

A cardiac cycle (or heartbeat) comprises one sequence of systole anddiastole. It can be detected by a physician counting the patient's pulserate. It is also reflected by the cardiac rhythm as recorded on anelectrocardiogram. The electrocardiogram (ECG) records the electricalactivity of the heart as seen on the surface of the body. The electricalactivity corresponds to the electrical cardiac depolarization in eitherthe atrium and/or ventricle. On the ECG, the atrial depolarization isrepresented by a waveform referred to as the P-wave, while theventricular depolarization is represented by a waveform referred to asthe QRS complex, sometimes abbreviated as an "R-wave."

A normal heart rate varies between 60 to 100 heartbeats (or cardiaccycles) per minute with an average of 72 bpm resulting in approximately100,000 cardiac cycles per day. The heart rate normally increases duringperiods of stress (physical or emotional) and slows during periods ofrest (sleep).

The amount of blood that the heart pumps in one minute is called thecardiac output. It is calculated by the amount of blood ejected witheach heartbeat (stroke volume) multiplied by the number of heartbeats ina minute. If the heart rate is too slow to meet the physiologicalrequirements of the body, the cardiac output will not be sufficient tomeet the metabolic demands of the body. One of two major symptoms mayresult. If the heart effectively stops with no heartbeat, there will beno blood flow and if this is sustained for a critical period of time (10to 30 seconds), the individual will faint. If there is a heartbeat butit is too slow, the patient will be tired and weak (termed low cardiacoutput).

Too slow a heartbeat is termed a bradycardia. Any heart rate below arate of 60 bpm is considered a bradycardia, however bradycardia onlyneeds to be treated if it is a persistent abnormality and causes apatient to have symptoms. In such cases, implantation of a permanentelectronic pacemaker is often prescribed.

An electronic pacemaker may also be referred to as a pacing system, or acardiac pacemaker. The pacing system is comprised of two majorcomponents. One component is a pulse generator that includes electroniccircuitry and a power cell or battery. The other is a lead or leadswhich connect the pulse generator to the heart.

Electronic pacemakers are described as either single-chamber ordual-chamber systems. A single-chamber system stimulates and senses thesame chamber of the heart (atrium or ventricle). A dual-chamber systemstimulates and/or senses in both chambers of the heart (atrium andventricle). The electronic pacemaker delivers an electrical stimulus tostimulate the heart to contract when the patient's own spontaneouspacemaker (i.e., the sinus node) fails or when conduction of an R-waveis blocked. In this way, the electronic pacemaker can help to stabilizethe heart rate of a patient's heart.

Conduction of an R-wave can be blocked in a variety of ways. Forexample, the atrio-ventricular (AV) node may be partially or completelyinsensitive to the propagation of a P-wave. Alternately, the Bundle ofHis or a bundle branch may suddenly stop propagation of the R-wave tothe ventricular tissue. Hereinafter, a P-wave which originates in thesinus node shall be referred to as a "spontaneous P-wave," a naturallyoccurring R-wave which is triggered by either a spontaneous or pacedP-wave shall be referred to as a "conducted R-wave," and the AV node,the Bundle of His, etc. shall be referred to as the heart's "conductionsystem."

Most pacemakers are referred to as demand-type pacemakers. This meansthat they are capable of sensing the electrical signal in or on thecardiac chamber by way of the pacing lead, which is placed in or on thechamber. The electrical signal as recorded in or on the heart is calledan electrogram (EGM), or sometimes an intracardiac electrogram (IEGM),and is a relatively large signal with very rapid changes in electricalpotential. The most rapid portion of this signal is called the intrinsicdeflection (ID), which is what is sensed by the pacemaker. Althoughmedical personnel commonly talk about pacemakers sensing P-waves orR-waves, this is not technically correct. The P-wave and R-wave,technically, are recorded from the surface of the body. The pacemaker,in contrast, senses the atrial or ventricular intrinsic deflection (ID)portion of the atrial or ventricular electrogram from within the heart.The atrial EGM coincides with the P-wave of the surface ECG while theventricular EGM coincides with the R-wave of the surface ECG. Thus, theterms P-wave and R-wave are commonly used, and will be used herein,synonymously with the atrial and ventricular intrinsic deflectionportions of the atrial and ventricular electrograms.

One of the parameters of the pacemaker that can commonly be programmedor set by the physician is a base rate, which is the lowest heart ratethat can be detected in a patient before the pacemaker will beginpacing. If the patient's ventricular heart rate is faster than this baserate, the pacemaker will recognize the ventricular electricaldepolarization and be either inhibited or triggered depending upon howthe electronic pacemaker is configured (and will reset its varioustiming cycles). If the patient's ventricular heart rate slows below thebase rate of the electronic pacemaker, the electronic pacemaker's timerswill expire (or "time out") and will cause the electronic pacemaker toperiodically release an output pulse (electrical stimulation) at thebase rate, thus preventing the patient's ventricular heart rate fromfalling below the base rate.

The interval between consecutive output pulses within the same chamberis termed the automatic interval or basic pacing interval. The intervalbetween a sensed event and the ensuing paced event is called an escapeinterval. In single-chamber pacing systems, the automatic and escapeintervals are commonly identical. In dual-chamber pacing systems, thebasic pacing interval is divided into two sub-intervals. The intervalfrom a sensed R-wave or ventricular paced event to the atrial pacedevent is called an atrial escape interval. The interval from the sensedP-wave or atrial paced event to the ventricular paced event is calledthe AV interval (AVI), or AV delay.

In the majority of individuals, the most effective heartbeat istriggered by the patient's own spontaneous pacemaker. The electronicpacemaker is intended to fill in when the patient's spontaneouspacemaker fails or when the heart's conduction system fails. The firstpacing mode that was developed was single-chamber ventricularstimulation. It was soon recognized that this resulted in the loss ofappropriate synchronization between the atria and ventricles in whichcase, the hemodynamic efficiency of the heart was compromised and thecardiac output fell despite maintaining an adequate rate. In thosepatient's whose need for a pacemaker was intermittent, with a normalrhythm occurring between times when pacing support was required,electronic pacemakers were developed which were set to a slow base rate.This allowed the patient's underlying rhythm to slow to this very lowbase rate before the electronic pacemaker would be activated. While thepatient would be protected from a systole (a total absence of anyheartbeat), the loss of appropriate AV synchrony combined with the slowrate was often hemodynamically inefficient, i.e., the efficiency of theheart as a pump was compromised.

One approach to remedying this inefficiency utilizes a hysteresiscircuit, in which the hysteresis escape rate of the pacemaker is slowerthan the automatic rate. When the hysteresis circuit was invoked, thepatient's underlying cardiac rhythm is permitted to persist until theheart rate falls below a hysteresis escape rate. When this happens,there is one cycle of pacing at the hysteresis escape rate followed bypacing at a more rapid rate until a conducted R-wave is sensed. When theconducted R-wave is sensed, the hysteresis escape rate is restored toagain inhibit the pacemaker and allow underlying cardiac rhythm topersist.

A number of heretofore unsolved problems exist with hysteresis. Oneproblem is confusion on the part of the medical personnel caring for thepatient as to why the patient's underlying rhythm occurs at a slowerrate than the automatic rate to which the pacemaker is set. A secondproblem is that the slow atrial escape rate promotes the occurrence ofpremature ventricular contractions (PVC's), ectopic beats, or pathologicR-waves. In operation, the electronic circuits of the pacemaker sense aPVC as an R-wave, and therefore assume that natural conduction hasreturned, and that therefore the pacing at the more rapid rate is nolonger needed. As a result, the escape interval of the pacemaker isreset to the slower hysteresis escape rate following each PVC, therebyeffectively maintaining a slower cardiac rate.

In view of the above problems, (confusion on the part of the medicalcommunity, and the repeated resetting of the pacemaker to a slower rateby the occurrence of PVC's) hysteresis was not well accepted by themedical community until such time as it was introduced as a programmableparameter capable of being enabled or disabled, and when enabled,capable of being adjusted so that the degree of hysteresis (i.e., thedifference between the slow and fast atrial escape rates) could bechanged.

Since the goal of hysteresis is to allow the patient's underlying rhythmwith appropriate AV synchrony for as much time as possible, whileproviding pacing support at a hemodynamically efficient rate at onlythose times when the patient requires such support, hysteresis was notincorporated into the first dual-chamber pacing systems, which weredesigned to always provide hemodynamically efficient AV synchrony. Somephysicians, however, recognized that some patients, whose heart rateprecipitously and abruptly slowed, not only needed a more rapid cardiacrate at these times, but they also required AV synchrony.Problematically, if the base rate of the typical dual-chamber pacemakeris programmed to a rate required when cardiac pacing is needed by suchpatients, the electronic pacemaker frequently controls the patient'srhythm even when cardiac pacing is not needed. In an attempt to solvethis problem, adaptations were introduced into some of the firstgeneration dual-chamber electronic pacemakers to allow hysteresis in theDDI mode. This allowed the electronic pacemaker to remain inhibited inthe presence of sensed cardiac signals, and to stimulate only when theelectronic pacemaker was needed. Once activated, the electronicpacemaker will then pace in both atrium and ventricle while tracking atthe atrial rate until such time as an R-wave was sensed, therebyinhibiting the generation of a V-pulse. Once inhibited, hysteresis wouldrequired expiration of the hysteresis escape interval before theelectronic pacemaker again released output pulses.

Several problems persist in the application of hysteresis in the DDImode. The first is that PVC's, a limitation first noted withsingle-chamber hysteresis systems, may be equally limiting in thedual-chamber pacing mode. The second is that when pacing is required,patients often need a relatively short AV interval for optimumhemodynamic function. However, setting a short AV interval could resultin the pacemaker usurping control of the heart's normal conductionsystem, resulting in sustained periods of pacing when it is no longerrequired. On the other hand, setting a long AV interval, while resultingin appropriate pacing system inhibition when pacing therapy is notrequired, may allow AV conduction when pacing is required, causingrepeated reinitiation of the longer hysteresis escape interval.Consequently, sustained pacing at the relatively slow hysteresis escaperate may result, which (while appropriate for a few cardiac cycles) maynot be appropriate for sustained periods of pacing. An electronicpacemaker that overcomes these problems would be highly desirable.

An additional problem exists when hysteresis is utilized in an AAI(R)mode of operation, namely, AAI(R) pacemaker syndrome. AAI(R) pacemakersyndrome exists when the heart rate (i.e., atrial paced rate) increasesor an A--A interval shortens (whether due to the programmed automaticrate or under rate responsive sensor drive), but the A-R interval doesnot shorten. As a result, an atrial output pulse (A-pulse), which causesan atrial contraction, is generated coincident with the precedingconducted ventricular contraction (R-wave). When this occurs, the atriumcontracts against a closed A-V valve, and therefore, is unable to forceblood into the ventricle (which is also contracting). As a result, poorhemodynamic efficiency is achieved.

From the above, it is evident that improvements in the use of hysteresisin dual-chamber pacemakers are needed and desirable.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing aspecial type of AV/PV hysteresis in a dual-chamber pacemaker wherein along AV delay (or AVI) is provided, thereby affording as muchopportunity as possible for natural AV conduction to occur, but wherethe AV delay is automatically shortened during AV block. Once the AVdelay is automatically shortened, the invention periodically searchesfor a return of AV conduction (absence of AV block) so that the AV delaycan be returned to its longer value as soon as possible, therebyoptimizing hemodynamic efficiency.

In accordance with one aspect of the invention, the pacemaker circuitsdetermine or "learn" the natural conduction time of the patient. This isdone by monitoring or measuring the "AR interval" (where the AR intervalis the time interval between an atrial event, either a P-wave or anA-pulse, and a subsequently conducted R-wave) over a specified period oftime. Once learned, the natural conduction time is used as a referenceagainst which subsequently measured AR intervals are compared to betterdistinguish conducted ventricular contractions from ectopic, pathologic,or other nonconducted ventricular contractions (e.g., PVC's). If themeasured AR interval is approximately the same as the "learned" naturalconduction time, then the R-wave at the conclusion of the measured ARinterval is presumed to be a conducted R-wave that signals the return ofAV conduction. Once AV conduction returns, in accordance with the AV/PVhysteresis system employed by the invention, the AV delay is lengthenedto its original value. If, on the other hand, the measured AR intervalis significantly different than the "learned" natural conduction time,then the R-wave at the conclusion of the measured AR interval ispresumed to be a nonconducted R-wave, and the AV delay is kept short.

In accordance with other aspects of the invention, other techniques maybe used to verify the return of AV conduction, in stead of, or inaddition to, the monitoring or "learning" of the natural conduction time(AR interval) as described above. Such other techniques include, e.g.,defining a programmable time window within the cardiac cycle thatdefines when an R-wave that occurs during the cardiac cycle is one thatevidences the return of AV conduction; measuring the amplitude of asensed R-wave to determine if such amplitude evidences the return of AVconduction; examining the morphology (shape) of a sensed R-wave todetermine if it is indicative of the return of AV conduction; and/orrequiring two or three or more consecutive occurrences of an R-waveduring consecutive cardiac cycles before making a decision as to whetherAV conduction has returned. Indeed, any technique that aids inidentifying whether AV conduction is present or absent may be used withthe invention.

In the above manner, the benefits of hysteresis pacing are achieved,while at the same time the problems that have plagued prior arthysteresis systems are minimized. For example, the continual resettingof the AV delay to its longer value, caused by PVC'S, or othernonconducted ventricular contractions, as commonly occur in patientsfitted with electronic pacemakers, is avoided. This is because only anR-wave at the conclusion of a measured AR interval that matches a"learned" natural conduction time, or that otherwise evidences thereturn of AV conduction, allows the AV delay to be reset back to itslonger value in the dual-chamber pacing system when programmed to a longAV delay.

In accordance with another aspect of the invention, for use withpacemakers having rate-responsive features, the amount by which the AVdelay or AVI is changed may be keyed to the sensor-indicated rate of thepacemaker. This effectively prevents the AAI(R) pacemaker syndrome fromoccurring.

It is thus a feature of the present invention to provide an implantablepacemaker that automatically sets its AV interval to either a long orshort value in order to optimize the hemodynamic performance of theheart with which the pacemaker is used.

It is another feature of the invention to provide such setting of the AVinterval while avoiding the AAI(R) pacemaker syndrome, i.e., preventingthe issuance of an A-pulse on top of an R-wave (including the S-T andthe T-waves), thereby assuring that the atria do not contract at a timewhen the atrial-ventricular valves are closed.

It is yet another feature of the invention to provide an automatic AVshortening procedure. Such procedure may be automatically invoked toshorten the AV interval (e.g., whenever AV block occurs), and maythereafter automatically search for the return of AV conduction (e.g.,whenever a conducted R-wave is sensed). Alternatively, such searchingmay occur in accordance with a prescribed schedule (e.g., every xcardiac cycles, where x is an integer greater than eight), or pursuantto some other defined schedule. Once AV conduction returns, the AVinterval is then reset to its original (longer) value.

It is an additional feature of the invention to provide positive AVhysteresis within a dual-chamber pacemaker wherein several differenttechniques may be used to verify the presence or absence of AVconduction, and therefore to signal when the AV interval should beswitched between a short value and a longer value.

It is still another feature of the invention, in accordance with oneembodiment thereof, to provide a atrial-based dual-chamber pacemaker,and method of operating such pacemaker, wherein the overall A-to-Ainterval of the pacemaker remains unchanged even through the AV intervaldoes change between long and short values.

It is another feature of the invention to provide ventricular support ina dual-chambered pacemaker operating in an AAI mode only when it isneeded (i.e., only when AV block is detected), so as to minimizeunnecessary power consumption by the cardiac pacemaker, and to terminatethe ventricular support response when there is AV conduction.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of an electronic dual-chamber electronicpacemaker capable of operating in accordance with the invention;

FIG. 2 is an illustration of an exemplary timing/event diagram thatdepicts the cardiac and pacemaker events that transpire when thepacemaker of FIG. 1 detects the need for ventricular support pacing;

FIG. 3 is an illustration of an exemplary timing/waveform diagram as inFIG. 2 that depicts the cardiac and pacemaker events that transpire whenthe pacemaker of FIG. 1 carries out a conducted R-wave "scan" operationand determines that ventricular support pacing is no longer needed;

FIG. 4 is an illustration of an exemplary timing/waveform diagram as inFIG. 2 that depicts the cardiac and pacemaker events that transpire whenthe pacemaker of FIG. 1 carries out a conducted R-wave "scan" operationand determines that ventricular support pacing continues to be needed;

FIG. 5 is a flow chart of the steps traversed by the electronicpacemaker of FIG. 1 in order to initiate and terminate the ventricularsupport response as described in reference to FIGS. 2, 3 and 4;

FIG. 6 is an illustration of an exemplary timing/waveform diagram as inFIGS. 2-4 that depicts the cardiac and pacemaker events that transpirewhen the pacemaker of FIG. 1 detects a premature ventricular contraction(PVC);

FIG. 7 is an illustration of an exemplary timing/waveform diagram as inFIGS. 2-4 that depicts the cardiac and pacemaker events that transpirewhen the pacemaker of FIG. 1 detects the need for an increased heartrate;

FIG. 8 is an illustration of an exemplary timing/waveform diagram as inFIGS. 2-4 that depicts the cardiac and pacemaker events that transpireas the pacemaker of FIG. 1 "learns" an average AR interval of a patient;

FIG. 9 depicts an exemplary timing/waveform diagram as in FIGS. 2-4 thatdepicts the cardiac and pacemaker events that transpire as the pacemakerof FIG. 1 utilizes the average AR interval learned in FIG. 8 todistinguish between, e.g., premature ventricular contractions andconducted ventricular contractions;

FIG. 10 is a block diagram of a pacing system that depicts, inaccordance with a preferred embodiment of the invention, the mainhardware components of the implantable pacemaker of FIG. 1;

FIG. 11 is a block diagram of the analog chip portion of the pacemakerof FIG. 10;

FIG. 12 is a block diagram of the digital chip portion of the pacemakerof FIG. 10, and illustrates the use of a microprocessor to control theoperation of the pacemaker;

FIG. 13 is a timing/waveform diagram that depicts a specific example ofpositive AV hysteresis with atrial based timing in accordance with thepresent invention; and

FIG. 14 is a timing/waveform diagram that depicts a specific example ofpositive AV hysteresis with ventricular-based timing in accordance withthe present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

As indicated above, the present invention is directed to method ofoperating an implantable dual-chamber pacemaker that automatically setsor adjusts the AV interval (or PV interval) of the pacemaker to a longvalue or a short value, thereby providing a type of dual-chamberhysteresis. If AV conduction is present (i.e., if a conductedcontraction of the ventricle is sensed, as evidenced by the presence ofan R-wave which regularly follows an atrial contraction, either aspontaneous P-wave or a paced atrial contraction), then the longer AV(or PV) interval is used, thereby preserving the natural hemodynamics ofthe heart and conserving the limited energy of the pacemaker's battery.If AV block exists (as determined by the lack of an R-wave during thelonger AV interval), then a V-pulse is generated to pace the ventricle,and the shorter AV interval is used thereafter.

On a regular basis (e.g., after a fixed period of time or a fixed numberof cardiac cycles), a determination is made as to whether AV conductionhas returned. If AV conduction has returned, the longer AV interval isagain reinstated. If AV block persists, then the shorter AV intervalcontinues to be used. In a rate-responsive pacemaker, both the longerand shorter AV intervals may be keyed to the sensor-indicated rate,i.e., these values change proportionate to the change in thesensor-indicated rate, so as to avoid the occurrence of the AAI(R)pacemaker syndrome.

Throughout the discussion that follows, reference will frequently bemade to the AV interval ("AVI"). It is to be understood that all suchreferences to the AV interval also apply to the PV interval, and thatwhether the AV or PV interval is used depends upon the particular typeof atrial activity--an A-pulse or a P-wave-- that starts the AV (or PV)interval. Similarly, it is to be understood that any references made tothe PV interval also apply to the AV interval. It is further to beunderstood that when the PV interval is used, it will typically be (butdoes not have to be) shorter than the AV interval by a prescribedamount, e.g., 20-40 msec, to account for the latency time involvedbetween applying an A-pulse and having the atrial tissue respond with adepolarization. Those of skill in the art can readily fashionappropriate circuitry to utilize either an AV interval or a PV interval,whichever applies to a given cardiac cycle. For the discussion thatfollows, then, where reference is made to the AV interval, such AVinterval should be considered as the time interval between atrialchannel activity, whether such atrial channel activity comprises anA-pulse or a P-wave, and the subsequent delivery of a ventricularstimulation pulse (V-pulse).

Advantageously, the present invention may be implemented using a widevariety of dual-chamber pacemaker configurations and pacemaker hardware.Any pacemaker configuration that allows the pacemaker AV (or PV)interval to be automatically set to a desired value (e.g., a long valueor a short value) may be used to implement the invention. Thedescriptions that follow are only exemplary of a few of suchconfigurations.

In FIG. 1, there is illustrated a functional block diagram of adual-chamber pacemaker 10. Such functional diagram will be used to teachthe primary functions carried out by the pacemaker 10. A preferredembodiment of the actual hardware and components used within thepacemaker 10 to carry out the pacemaker functions will then be describedin conjunction with FIGS. 10-12. Next, techniques or methods that may beused by the pacemaker 10 to implement the present invention will bedescribed in conjunction with the flow diagrams and timing/eventdiagrams of FIGS. 2-9.

As shown in FIG. 1, the pacemaker 10 is coupled to a heart 12 by way ofleads 14 and 16. (Note, in subsequent figures, e.g., FIG. 10, the leads14 and 16 are referred to as the lead system 19.) The lead 14 has anelectrode 15 that is in contact with one of the atria of the heart, andthe lead 16 has an electrode 17 that is in contact with one of theventricles of the heart. The leads 14 and 16 carry stimulating pulses tothe electrodes 15 and 17 from an atrial pulse generator (A-PG) 18 and aventricular pulse generator (V-PG) 20, respectively. Further, electricalsignals from the atria are carried from the electrode 15, through thelead 14, to the input terminal of an atrial channel sense amplifier(P-AMP) 22; and electrical signals from the ventricles are carried fromthe electrode 17, through the lead 16, to the input terminal of aventricular sense channel amplifier (R-AMP) 24.

Controlling the dual-chamber pacemaker 10 is a control circuit orcontrol system 26. The control system 26 receives the output signalsfrom the atrial amplifier 22 over signal line 28. Similarly, the controlsystem 26 receives the output signals from the ventricular amplifier 24over signal line 30. The output signals on signal lines 28 and 30 aregenerated each time that a P-wave or an R-wave is sensed within theheart 12. The control circuit or system 26 also generates triggersignals that are sent to the atrial pulse generator 18 and theventricular pulse generator 20 over signal lines 32 and 34,respectively. These trigger signals are generated each time that astimulation pulse is to be generated by the respective pulse generator18 or 20. A stimulation pulse generated by the A-PG 18 is referred to asthe "A-pulse," and the stimulation pulse generated by the V-PG 20 isreferred to as the "V-pulse." During the time that either an A-pulse orV-pulse is being delivered to the heart, the corresponding amplifier,P-AMP 22 and/or R-AMP 24, is typically disabled by way of a blankingsignal presented to these amplifiers from the control system over signallines 36 and 38, respectively. This blanking action prevents theamplifiers 22 and 24 from becoming saturated from the relatively largeA-pulse or V-pulse, respectively, that is present at the input terminalsof such amplifiers during this time. Such blanking action also helpsprevent residual electrical signals present in the muscle tissue as aresult of the pacemaker stimulation from being interpreted as P-waves orR-waves.

As shown in FIG. 1, the pacemaker 10 also includes a memory circuit 40that is coupled to the control system 26 over a suitable data/addressbus 42. The memory circuit 40 allows certain control parameters, used bythe control system 26 in controlling the operation of the pacemaker, tobe programmably stored and modified, as required, in order to customizethe pacemaker's operation to suit the needs of a particular patient.Such data includes the basic timing intervals used during operation ofthe pacemaker, such as the programmed atrial escape interval (AEI). Forpurposes of the present invention, such data may also include a familyof AV interval data that may be retrieved during an adjustment sequenceof the AV interval, as explained more fully below. Further, data sensedduring the operation of the pacemaker may be stored in the memorycircuit 40 for later retrieval and analysis.

A telemetry circuit 44 is further included in the pacemaker 10. Thistelemetry circuit 44 is connected to the control system 26 by way of asuitable command/data bus 46. In turn, the telemetry circuit 44, whichis included within the implantable pacemaker 10, may be selectivelycoupled to an external programming device 48 by means of an appropriatecommunication link 50, which communication link 50 may be any suitableelectromagnetic link, such as an RF (radio frequency) channel.Advantageously, through the external programmer 48 and the communicationlink 50, desired commands may be sent to the control system 26.Similarly, through this communication link 50 and the programmer 48,data (either held within the control system 26, as in a data latch, orstored within the memory circuit 40), may be remotely received from thepacemaker 10. In this manner, noninvasive communications can beestablished from time to time with the implanted pacemaker 10 from aremote, non-implanted location. Many suitable telemetry circuits knownin the art that may be used with the present invention for the telemetrycircuit 44. See, e.g., U.S. Pat. No. 4,847,617, incorporated herein byreference.

The pacemaker 10 in FIG. 1 is referred to as a dual-chamber pacemakerbecause it interfaces with both the atrial and the ventricular chamberof the heart. Those portions of the pacemaker 10 that interface with theatria, e.g., the lead 14, the P-wave sense amplifier P-AMP 22, theatrial pulse generator A-PG 18, and corresponding portions of thecontrol system 26, are commonly referred to as the atrial channel.Similarly, those portions of the pacemaker 10 that interface with theventricle, e.g., the lead 16, the R-wave sense amplifier R-AMP 24, theV-pulse generator V-PG 20, and corresponding portions of the controlsystem 26, are commonly referred to as the ventricular channel.

Throughout the discussion that follows, frequent reference will be madeto "atrial activity," "atrial events," "ventricular activity," or"ventricular events." Atrial activity or an atrial event thus compriseseither the sensing of a P-wave by the sense amplifier P-AMP 22, or thegenerating of an A-pulse by the A-pulse generator A-PG 18. Similarly,ventricular activity or a ventricular event comprises either the sensingof an R-wave by the sense amplifier R-AMP 24 or the generation of aV-pulse by the V-pulse generator V-PG 20.

In some pacemakers that implement the present invention, the pacemaker10 may further include one or more physiological sensors 52 that is/areconnected to the control system 26 of the pacemaker over a suitableconnection line 54. While the sensor 52 is illustrated in FIG. 1 asbeing included within the pacemaker 10, it is to be understood that thesensor may also be external to the pacemaker 10, yet still be implantedwithin or carried by the patient. A common type of sensor is an activitysensor, such as a piezoelectric crystal, mounted to the case of thepacemaker. Other types of physiologic sensors, such as sensors thatsense the oxygen content of blood, respiration rate, pH of blood, bodymotion, and the like, may also be used in lieu of, or in addition to, anactivity sensor. The type of sensor, if any, used is not critical to thepresent invention. Any sensor or combination of sensors capable ofsensing some physiological parameter relatable to the rate at which theheart should be beating can be used. A pacemaker using such sensors iscommonly referred to as a "rate-responsive" pacemaker because such apacemaker adjusts the rate (escape interval) of the pacemaker in amanner that tracks the physiological needs of the patient. Further, suchsensors, and the circuitry used therewith, generate what is usuallyreferred to as a "sensor-indicated rate" (SIR) signal, which signaldefines the minimum rate at which the heart should be beating given thephysiological activity or other parameters sensed by the sensors.

Referring next to FIG. 10, there is shown a preferred configuration of apacing system made in accordance with the present invention. The systemincludes the external programmer 48, the implantable pacemaker 10, andthe lead system 19. The lead system 19 includes conventional atrial andventricular leads and electrodes, as described previously. The leadsystem 19 may also include an oxygen sensor lead, which lead typicallycontains an LED-detector assembly used to measure the oxygen content ofthe blood. Such a lead is described, e.g., in U.S. Pat. No. 4,815,469,incorporated herein by reference.

The external programmer 48 includes a telemetry head 49 that ispositioned proximate the implantable pacemaker 10 whenever thecommunication link 50 is to be established between the pacemaker 10 andthe external programmer 48. The external programmer may be ofconventional design, as described, e.g., in U.S. Pat. No. 4,809,697,incorporated herein by reference.

The components of the pacemaker 10 are housed within a suitable sealedcase or housing 400 (which case or housing is represented in FIG. 10 bythe dashed line 400). The case 400 is preferably a titanium metal case.The components within the case 400 include an RF coil 402, a memory chip404, a battery 406, one or more sensors in a sensor circuit 408, acrystal 410, an output/protection network 412, an analog chip 420 and adigital chip 440.

The battery 406, which is by volume the largest component within thepacemaker 10, may be of conventional design, and is a lithium batterythat provides operating power to all of the electronic circuits withinthe pacemaker. The RF coil 402 is used to establish the communicationlink 50 with the telemetry head 49. The crystal 410 is used inconjunction with a crystal oscillator circuit on the digital chip 440(described below) to provide a stable clock frequency for the pacemakercircuits. For the embodiment shown in FIG. 10, the frequency of thecrystal oscillator is 32 KHz, although any suitable frequency could beused. The sensor circuit 408 includes appropriate sensors used by thepacemaker as it carries out a rate-responsive pacing function. Forexample, in one embodiment, the sensor circuit 408 includes anaccelerometer adapted to sense patient activity.

The memory chip 404 is a low-power static random access memory (RAM)chip wherein the operating parameters, e.g., control variables, of thepacemaker may be stored, and wherein sensed data may be stored, asrequired. The analog chip 420 and the digital chip 440 contain the mainprocessing and control circuits of the pacemaker. These chips areadvantageously designed to minimize the number of components neededexternal thereto for operation of the pacemaker. The analog chip 420interfaces with the lead system 19 through the output and protectionnetwork 412, which network includes output capacitors, appropriatefeed-through connectors to allow electrical connection through thehermetically sealed case, and the like, as are commonly used inimplantable medical devices.

In FIG. 11, a block diagram of the analog chip 420 is shown. The analogchip 420 contains all the necessary subsystems and modules to interfaceto the lead system 19 and the digital chip 440. For example, astartup/bias-current/reference module 422 contains the power-up signalsused to initialize the pacemaker circuit when the battery is firstapplied. A low battery module 424 detects four voltage levels of thebattery voltage for determining the battery status. A case amplifier 426generates a CASE bias voltage that is used as a reference for the senseand IEGM (intra cardiac electrogram) amplifier module 428. The module428 includes the P-wave amplifier 22 and the R-wave amplifier 24,described above in FIG. 1. A measured data module 430 measures thebattery voltage and current and other analog parameters of the pacingsystem. An ADC and Logic module 432 includes an analog-to-digitalconverter (ADC) and timing logic that are used to convert the analogsignals of the pacemaker to 8-bit digital words. These digital words arethen passed to a digital module 434, which module is used to generateall the basic timing and bus control functions as data is passed backand forth between the analog chip 420 and the digital chip 440.

As shown in FIG. 11, it is seen that a Runaway Protection (RAP) circuitoscillator 436 is also coupled to the Digital Module 434. Suchoscillator 436 provides an independent time base for limiting thehighest pacing rate allowed by the pacemaker. Further coupled to thedigital module 434 is the sensor circuit 408. The sensor circuit 40includes appropriate sensors for sensing activity and other parameters.For example, an 02 sensor circuit 409 may be used in conjunction withthe oxygen sensor lead, when used, to measure blood oxygen of thepatient. An activity sensor 408 may also be used to sense patientactivity as measured, e.g., by an accelerometer. A charge pump circuit438 generates the output voltages for the stimulation pulses that aredelivered to the patient's heart. A network of output switches 439connects the charge developed by the pump circuit 438 to the outputleads at the appropriate time to form the appropriate stimulationpulses.

It is thus seen that the analog chip 420 contains the necessarycircuitry to sense and detect atrial or ventricular events, digitizeIEGM waveforms, measured data and other various analog signals, andprovide such sensed and digitized signals to the digital module 434 foruse by the digital chip 440. The charge pump circuit 438 acts as avoltage doubler/tripler for high output pulse capability. The outputpulse width is controlled by the output switches 439. The condition ofthe battery is monitored, and independent Runaway Protection isprovided.

In FIG. 12, it is seen that the main control element of the pacemaker isa microprocessor 442, which microprocessor is included within thedigital chip 440. The digital chip 440 contains all the necessary logicto interface the analog chip 420 with the internal microprocessor 442.The microprocessor 442 includes a basic CPU (central processing unit)and 8K of static RAM (random access memory). In addition, an 8K by 8KRAM 446 is connected to the microprocessor 442 to store data andprograms. Microprocessor support logic 444, also coupled to themicroprocessor 442, includes interrupt logic, timer logic, noise/sensedevent logic, and magnet status logic. A bus controller 448 is furtherincluded on the digital chip 440 to provide DMA timing and control ofdata transfer with the analog chip 420, including logic timing andcontrol of the analog-to-digital converter 432 (FIG. 11) and telemetrydata. Telemetry channel logic 450 contains clock logic, IEGM and markerlogic, telemetry command protocol logic, telemetry interrupt logic,error checking logic and CPU reset logic. An RF transceiver 452, coupledto the RF coil 402, transmits and receives telemetry data from theexternal programmer 48 through the telemetry head 49 (see FIG. 10). Acrystal oscillator circuit 456, in conjunction with the crystal 410(external to the digital chip 440) provides the crystal time base of thepacemaker system. A current generator 454 provides the bias currents forthe digital chip. A reed switch circuit 458 detects the presence of amagnetic field, which magnetic field is present whenever the telemetryhead 49 is in place on the patient's skin above the location where thepacemaker is implanted.

The pacemaker circuitry described in connection with FIGS. 10-12 aboveprovides the basic functions of the pacemaker described in connectionwith FIG. 1, plus other pacing/sensing functions as are known in theart. For purposes of the present invention, the pacemaker circuitry ofFIGS. 10-12 sets the basic timing of the pacing interval, includingsetting an AV interval and a VA interval. The circuitry also providesfor sensing or detecting ventricular events (R-waves) and/or atrialevents (P-waves), and for measuring and "learning" the time intervalbetween a sensed or paced atrial event and a conducted ventricular event(R-wave). Such AR/PR time interval, as indicated previously, comprisesthe natural conduction time of the patient's heart. In accordance withthe present invention, the AV interval is initially set to a long valuegreater than the natural conduction time. The AV interval remains atsuch long value for so long as AV conduction is present, i.e., for solong as an R-wave is sensed before the timing out of the AV interval. Ifan R-wave is not sensed before the timing out the long AV interval, thenthat indicates that AV block is present, and the AV interval isshortened to its shorter value to assure that needed ventricular supportis provided. Meanwhile, the natural conduction time, once learned, issaved or stored as a reference value. Such reference value is then usedas a standard or reference against which subsequent measured ARintervals are compared to determine if the R-wave that concludes suchsubsequent AR intervals is a conducted R-wave, which signals the returnof AV conduction, or an ectopic, pathologic, or other early R-wave, thatdoes not signal the return of AV conduction.

In some embodiments of the invention, the pacemaker circuitry mayinclude means for measuring the amplitude of a sensed R-wave, and/orexamining the morphology of a sensed R-wave, and for comparing suchmeasured or examined amplitude/morphology with previously definedamplitudes and/or morphologies indicative of the presence or absence ofAV conduction. Further, timing circuitry may be employed that defines aprogrammable time window synchronized with the cardiac cycle duringwhich a sensed R-wave suggests the return of AV conduction.

If AV conduction has returned, then the AV interval is reset to itslonger value. If AV conduction has not returned, then the AV intervalremains at its shorter value. In this way, then, the pacemaker's AV (orPV) interval automatically is set to a longer or shorter value in orderto maintain appropriate hemodynamics to maximize cardiac output for agiven patient.

It is noted that in addition to the embodiment of the inventionillustrated in FIGS. 10-12, still other embodiments of a control system26 may be utilized. The embodiment described above in FIGS. 10-12 showsa control system and pacemaker configuration that is based on amicroprocessor. Another representative microprocessor-based system isdescribed, for example, in U.S. Pat. No. 4,940,052, entitled"Microprocessor Controlled Rate-Responsive Pacemaker Having AutomaticThreshold Adjustment," incorporated herein by reference. The controlsystem may also be based on a state machine wherein a set of stateregisters define the particular state of the pacemaker at any instant intime. As is known in the art, state machines may be realized usingdedicated hardware logic circuits, or a suitable processor(programmed-controlled circuit) to simulate such dedicated hardwarelogic circuits. However implemented, the results are the same--the stateof the pacemaker is defined at any instant of time by the pacemakerlogic and sensed events which transpire or fail to transpire, such asthe sensing of an R-wave, or the timing out of a timer. A description ofbasic state machine operation may be found, e.g., in U.S. Pat. No.4,712,555. Reference is also made to U.S. Pat. No. 4,788,980, whereinthe various timing intervals used within the pacemaker and theirinterrelationship are more thoroughly described; and U.S. Pat. No.4,944,298, wherein an atrial-rate based programmable pacemaker isdescribed, including a thorough description of the operation of thestate logic used to control such a pacemaker. The '555, '980 and '298patents are also incorporated herein by reference.

The details of the control system 26, whether based on a microprocessor,state machine, or other type of control devices, or simulated controldevices, are not critical to an understanding or implementation of thepresent invention, and hence are not presented herein. Such details maybe found in the referenced applications and patents, if desired.

All that is important for purposes of this embodiment of the presentinvention is that the control system of the pacemaker be capable, inconjunction with other pacemaker circuitry, of: (1) measuring the ARinterval defined as a time interval between an atrial event (i.e., thegeneration of an A-pulse or the sensing of a P-wave) and a sensedR-wave; (2) averaging the AR interval over a prescribed number ofcardiac cycles to define a "learned" AR interval, the "learned" ARinterval corresponding to a natural conduction time of the heart; (3)determining whether an R-wave is naturally conducted by comparing the ARinterval with the "learned" AR interval; (4) shortening the AV intervalby a prescribed amount when AV conduction is not present, i.e., when AVblock occurs; and (5) setting the AV interval to a long or short valueas a function of whether AV conduction has returned or not,respectively.

The control system should also be capable of regularly checking,whenever the short AV interval is being used, to determine if AVconduction has returned by, e.g., (a) systematically increasing the AVinterval in small increments up to its longer value, (b) looking for theoccurrence an R-wave before the timing out of the AV interval, (c)measuring the AR interval associated with any R-wave that does occurbefore the timing out of the AV interval, and (d) testing the measuredAR interval of any R-wave that does occur against the "learned" ARinterval to determine if such R-wave represents the return of AVconduction;

The control system should also be capable, in conjunction withappropriate sensor circuitry, of adjusting the long and short values ofthe AR interval as a function of the sensor-indicated rate signal.

Note that the measurement or learning of the AR interval can be made inconventional manner, and may involve an averaging of the AR intervalover several cardiac cycles, or other computation or estimation of theAR interval as is known in the art. Once the AR interval has beenmeasured, or otherwise learned, the pacemaker then sets its AV (or PV)interval to a long or short value as described above.

For other embodiments of the invention, the control system should becapable, in conjunction with other pacemaker circuitry, of: (1) settingthe AV interval to a programmed initial value; (2) sensing whether anR-wave occurs during such AV interval, and if not (3) issuing a V-pulseat the conclusion of the AV interval and then shortening the AV intervalto a shorter value; (4) maintaining the AV interval at its shorter valuefor a prescribed number of cardiac cycles or for a prescribed time; (5)periodically (or in accordance with a prescribed schedule) scanning forthe return of AV conduction, and if AV conduction returns (6) returningthe AV interval to its initial (longer) value. A feature of theinvention is that the control system of such embodiments, in conjunctionwith other pacemaker circuitry, is configured to readily recognize thatthe return of AV conduction. AV conduction may be verified, e.g., bysimply monitoring the ventricular channel for the occurrence of one ormore R-waves, and monitoring or measuring such R-waves (e.g., bymeasuring/monitoring time of occurrence, number of consecutiveoccurrences, amplitude, and/or morphology) to determine if the sensedR-waves are indicative, or consistent with, the presence of AVconduction.

In FIG. 5, the above-described process--of determining the naturalconduction time (AR interval), and setting the AV (or PV) interval to ashort or long value as a function of whether AV conduction is present ornot-- is illustrated in a high level flow diagram. In FIG. 5, each mainstep of the process or sequence is shown as a "block" or "box," witheach block having a reference numeral assigned thereto to aid in theexplanation thereof. Such flowchart is particularly helpful when theinvention is implemented using a microprocessor, or equivalentprocessing device, that follows a stored program, with the flowchartrepresenting the stored program that is used by such processor.

Before describing the flow diagram of FIG. 5, however, it will behelpful to explain the invention in terms of the timing/event diagramsof FIGS. 2-4. Such timing/event diagrams illustrate the relevantcardiac/pacemaker "events" that occur over a sequence of several cardiaccycles, including the occurrence of P-waves, R-waves, the generation ofA-pulses and V-pulses, and whether the pacemaker is using a long orshort AV interval.

In FIG. 2, an illustration of a timing diagram is shown that depicts themain cardiac/pacemaker events that occur when the electronic pacemaker10 (FIG. 1) is operating in accordance with functional AAI modality anddetects, in accordance with the present invention, the need forventricular support pacing. As shown in FIG. 2, a first A-pulse 102 isfollowed by a first P-wave 104 and a first (conducted) R-wave 106. TheAV interval (AVI) is also depicted as a line 108 beginning at the firstA-pulse 102 and terminating with an arrowhead a prescribed period oftime (e.g., 300 ms) following the first A-pulse. In practice, theoriginal AVI may be a fixed value, hard coded into the electronicpacemaker; may be a value that is programmed transcutaneously into theelectronic pacemaker using the external programmer, or may be a valuethat is adapted by the electronic pacemaker, e.g., as a function of asensor-indicated rate. A dot interposed in the AVI line 108 before thearrowhead signifies the occurrence of the first R-wave 106 before thetermination of the AVI. In practice, such occurrence indicates to theelectronic pacemaker 10 that ventricular support is not needed. Similarnotation is used throughout FIGS. 2-4 and 6-9.

Note that throughout the timing/event diagrams described herein, it isassumed that atrial pacing is required, and therefore an A-pulse isshown at the beginning of each cardiac cycle. It will be understood,however, by one skilled in the art, that the teachings herein areequally applicable to situations wherein a natural, spontaneous P-wavesignal begins each cardiac cycle. Thus, reference to an A-pulse withinthe descriptions presented herein should be understood to apply to anyatrial event, to a naturally occurring P-wave or an A-pulse.

As shown in FIG. 2, a second AVI 110 is initiated in response to asecond A-pulse 112. The AVI 110 expires before the occurrence of aconducted R-wave. In response to such expiration, a first V-pulse 114 isgenerated, which is followed by a second (induced) R-wave. Thus, inaccordance with the teachings of the present invention, the expirationof the AVI 110 initiates a ventricular support response within theelectronic pacemaker. The ventricular support response includessubstituting a "short" AVI 116 for the original AVI 110. The short AVI116 may be, e.g., 200 ms in duration. In practice, the short AVI, likethe original (or longer) AVI, may be a fixed value, may be a value thatis programmable transcutaneously using e.g., the external programmer 48,or may be a value that is adapted by the pacemaker, e.g., as a functionof a sensor-indicated rate. Advantageously, such short AVI improves thehemodynamic efficiency of the heart, whenever ventricular support pacingis required, over the relative inefficiency that would result if thelonger AVI were used. The original (or longer) AVI, however, reduces theoccurrence of unnecessary ventricular stimulation (V-pulses). Thus, inthis way, the present invention dramatically improves upon heretoforeknown approaches that use ventricular support pacing in, e.g., a DDDmodality pacemaker functioning in the AR pacing state due to a longprogrammed AVI, i.e., a pacemaker that triggers the AVI upon thegeneration of an A-pulse or the detection of a P-wave.

In FIG. 2, it is seen that the short AVI is initiated in response tothird A-pulse 118 and a fourth A-pulse 120. V-pulses 122 and 124 aregenerated in response to each of these short AVI's expiring (timing out)before the occurrence of a conducted R-wave. The above-describedventricular support response, utilizing the short AVI, continues untilthe electronic pacemaker determines that such support is no longerneeded. The determination that ventricular support is no longer neededis made as depicted in FIGS. 3 and 4.

In FIG. 3, an exemplary timing/event diagram is illustrated to depictthe events that occur as the electronic pacemaker 10 (FIG. 1) carriesout an R-wave "scan" operation. Such "scan" operation is performed todetermine whether the ventricular support response (described in FIG. 2)continues to be needed. As can be seen in FIG. 3, the short AVI ispresumed to be present, and is initiated in response to first and secondA-pulses. In practice, the first and second A-pulses (in FIGS. 3 and 4)represent the last two A-pulses in a series of A-pulses following theinitiation of the ventricular support response. The first two A-pulsesin this series are the third and fourth A-pulses 118 and 120 shown inFIG. 2. The first and second A-pulses in FIG. 3 may be, e.g., the ninthand tenth A-pulses in this series. Thus, it is seen that the short AVI,once triggered, continues for a prescribed number of cardiac cycles. Theprescribed number of cycles may be relatively low, e.g., ten cardiaccycles, particularly when the short AVI is first triggered after a longperiod of having used the long AVI. Alternatively, the prescribed numberof cycles may be much higher, e.g., 128-512 cardiac cycles. In oneembodiment of the invention, a preferred number of cardiac cycles formaintaining the short AVI once triggered is 256.

Following the prescribed number of cardiac cycles, i.e., after thegeneration of the last A-pulse in the series (i.e., the second A-pulseof FIG. 3), the AVI is returned to its original (long) value in responseto the next A-pulse (i.e., A-pulse 126). The purpose of returning theAVI to its original (long) value is to scan for the occurrence of aconducted R-wave. As an alternative to abruptly lengthening the AVI toits longer value, the AVI may be progressively lengthened from its shortvalue to its long value over (e.g., 4-16 cardiac cycles). A cardiaccycle in which the original AVI is initiated, or in which the AVI islengthened progressively, following a series of cardiac cycles in whichthe short AVI is used, is referred to herein as a "scan" cycle. In thiscontext the original AVI, or progressively lengthened AVI, is used bythe electronic pacemaker to "scan" for the recurrence of a conductedR-wave after the ventricular support response has been initiated for aprescribed period.

Thus, as shown in FIG. 3, when the AVI is abruptly increased to itsoriginal value following A-pulse 126, a conducted R-wave 128 occursbefore the expiration of the lengthened AVI. The occurrence of R-wave128, once verified as a conducted R-wave (as opposed to an ectopic,pathologic, or other early R-wave), indicates that AV conduction hasreturned and that the ventricular support response is no longer needed.Hence, the ventricular support response is terminated, which means thatthe longer (original) AVI is used from that point forward. The longerAVI continues to be used until such time as a new determination is madethat AV block has returned (as described in reference to FIG. 2).

In FIG. 4, an exemplary timing diagram is illustrated to show the eventsthat occur when the pacemaker 10 (FIG. 1) carries out an R-wave "scan"and determines that the ventricular support response (described in FIG.2) continues to be needed. As can be seen in FIG. 4, the short AVI ispresumed to be present. As with FIG. 3, the first and second A-pulses ofFIG. 4 represent the last two A-pulses in the series of A-pulsesfollowing the initiation of the ventricular support response. Followingthe generation of the last A-pulse of the specified series, the AVIfollowing the next A-pulse 130 is returned to its original (or longer)value in order to "scan" for a conducted R-wave. When such a "scan" isperformed, unlike in the situation shown in FIG. 3, no conducted R-waveoccurs before the expiration of the longer AVI. The lack of a conductedR-wave indicates that the ventricular support response continues to beneeded. Hence, a V-pulse 131 is generated at the conclusion of thelonger AVI, and the next AVI, following A-pulse 132, is made short. Theshort AVI that follows the A-pulse 132 continues to be used followingeach subsequent A-pulse until it is again time to perform a "scan"operation in order to look for conducted R-waves. This process continuesuntil a conducted R-wave is detected during the original (longer) AVI.In this way, the invention periodically "scans" for the resumption ofconducted ventricular cardiac activity, while maintaining improvedhemodynamic efficiency in the heart during the intervening series ofcardiac cycles. The ventricular support response is terminated only whena conducted R-wave is detected during a scan cycle.

A key component of the invention described thus far relates to detectingor discerning a conducted R-wave that occurs during a "scan" (long AVI)operation, from an R-wave that is not a conducted R-wave, e.g., anectopic or pathologic R-wave. One technique used by the invention todiscriminate a conducted R-wave from a nonconducted R-wave is describedbelow in connection with FIGS. 6-9. Other techniques, in conjunctionwith or in addition to that shown in FIGS. 6-9, may also be used indiscerning a conducted R-wave form a nonconducted R-wave, as alsoexplained below.

In FIG. 5, a flow chart is shown of the main steps traversed by thecontrol system 26 of the electronic pacemaker 10 (FIG. 1) in order toinitiate and terminate a ventricular support response (as describedabove in connection with FIGS. 2-4). After carrying out an appropriateinitialization routine (such as are commonly known in the art of, e.g.,microprocessors and state-machines), the electronic pacemaker 10(FIG. 1) starts its operation (block 500) by setting the AVI to a longvalue (block 501). The AVI will be initiated in response to each atrialevent (where an "atrial event" comprises either an A-pulse generated bythe electronic pacemaker, as illustrated, for example, by the A-pulses102 and 112 of FIG. 2; or a naturally occurring P-wave). The electronicpacemaker also sets n to its low value, e.g., 4-16 (block 502), as willbe described in more detail below.

Once initialized, the electronic pacemaker then issues an A-pulse andbegins the AVI (block 503). A determination is then made as to whetheran R-wave occurs following the A-pulse and before the expiration of theAVI (block 504). In the event an R-wave is so detected (YES branch ofblock 504), the AVI interval is kept at its long value (block 501), n iskept at its low value (block 502), the next A-pulse is generated (at theappropriate time) and the next AVI begins (block 503). This process(blocks 501, 502, 503 and 504) repeats for so long as an R-wave isdetected before the expiration of the long AVI.

Should an R-wave not be detected (NO branch of block 504) before theexpiration of the AVI, indicating that at least first degree AV blockhas occurred, then a V-pulse is generated, and the AVI is shortened toits short value (block 506). Pacing then continues using the short AVIfor n cardiac cycles (blocks 522, 503, 504, 506, 508, etc.). That is, inaccordance with the programmed dual-chamber operation, an A-pulse isgenerated, followed by the short AVI. A V-pulse is generated at the endof the short AVI. After a prescribed atrial escape interval, anotherA-pulse is generated, and the process continues. Should an R-wave bedetected before the expiration of a short AVI, then such R-wave inhibitsthe generation of a V-pulse at the end of the AVI, but has no othereffect.

After n pacing cycles have elapsed using the short AVI (block 508),where n is a programmable number having a first-pass value of (e.g.,4-16 cycles, or more), then a "scan" operation begins (block 510). It isthe purpose of the "scan" operation to determine if AV conduction hasreturned. The scan operation commences by restoring the AVI to itsoriginal (long) AVI (block 510), either in one cycle or progressivelyover several cycles. If an R-wave is detected while "scanning," i.e.,during a lengthened AVI (YES branch of block 512), then a determinationis made (as explained below) as to whether such detected R-wave is aconducted R-wave (block 514). If so (YES branch of block 514), then theAVI is returned to its original (long) value (block 501), n is set atits low value (block 502), and normal pacing resumes using such long AVI(blocks 501, 502, 503, 504 et seq.).

If an R-wave is not detected while scanning (NO branch of block 512), orif a detected R-wave is not confirmed as a conducted R-wave (NO branchof block 514), then the AVI is set to its short value (block 518), thevalue of n is changed to an appropriate second-pass value, e.g., 64 to512, and pacing continues with ventricular support using such short AVI(block 522) for the prescribed number n of cycles (block 508). Note thatthe number n of cardiac cycles that must elapse using the short AVI thefirst time AV block is detected is not necessarily the same as thenumber of cycles that must elapse using the short AVI once a "scan"operation has been invoked but no conducted R-waves were detected. Thus,for example, before invoking the "scan" operation, n may be a relativelysmall number, e.g., 4-16. After a "scan" operation, however, n may be arelatively large number, e.g., 128-256. In this manner, once a sustainedAV block has been confirmed, and reconfirmed, a significant number ofcardiac cycles, and hence a non-trivial amount of time, must transpirebefore an additional scan operation is invoked. (In the descriptionabove, the suggested values for n are usually stated in terms of powersof two, 2^(i), for purposes of implementing in a digital counter,however, any convenient low and high value may be chosen.)

One of the steps in the process mentioned above (at block 514) relatesto confirming whether an R-wave was a conducted R-wave. Suchconfirmation is performed in order to assure that AV conduction hasreturned for more than just a transitory period of time. Typically, suchconfirmation is made by repeating the scan operation for a prescribednumber of cycles, e.g., 4-8 cycles. If an R-wave consistently occursover 4-8 consecutive scan cycles (i.e., cycles with a longer AVI), thenthat provides a good indication that AV conduction has in fact returned.Further, it is noted that, in general, requiring two or three or more(e.g., 4-8) consecutive occurrences of a prescribed event, e.g., thesensing of an R-wave, or the generation of a V-pulse, is one techniquethat may be used by the invention to confirm or verify that it is anappropriate time to change the AV interval (from long to short, or fromshort to long) or to take other action.

One of the reasons for confirming that an R-wave is in fact a conductedR-wave is to avoid having ectopic, pathologic, or other early R-waves(e.g., as occur during a PVC) from being falsely classified as conductedR-waves. That is, when conducted R-waves occur regularly, then suchregularity signals the return of AV conduction. The return of AVconduction, in turn, means that the pacemaker should respond by ceasingany ventricular support and lengthening the AVI back to its original(long) value. If an early (nonconducted) R-wave occurs, such R-wave doesnot signal the return of AV conduction. Hence, it is important that suchearly R-waves be distinguished from conducted R-waves so that thepacemaker can respond by either lengthening or not lengthening the AVI.

The present invention, in accordance with one embodiment thereof,distinguishes conducted R-waves from nonconducted R-waves by measuringand "learning" the AR interval, or natural conduction time of thepatient. Then, when an R-wave is subsequently detected, the AR timeinterval associated with the detected R-wave is compared with the"learned" natural conduction time. If the R-wave is a conducted R-wavethat evidences the return of AV conduction, the AR time intervalassociated with the detected R-wave will be roughly the same as thepreviously "learned"natural conduction time, e.g., within ±10-20%. Ifthe R-wave is not a conducted R-wave, however, but is an early R-wave,then the AR time interval associated with the detected R-wave will besignificantly different than the previously "learned" natural conductiontime. Thus, by simply "learning" the natural conduction time over asuitable number of cardiac cycles, and then using such learned naturalconduction time as a reference against which subsequent AR intervals canbe compared, a determination can be made as to whether the R-waveassociated with the subsequent AR interval is a conducted R-wave or not.

Using the learned time duration of the natural conduction time (ARinterval) as a reference against which subsequent AR intervals can becompared is just one way that the present invention can distinguishbetween conducted R-waves and nonconducted R-waves. Other techniquesthat may be used include monitoring the timing of when a sensed R-waveoccurs within the cardiac cycle. Such monitoring may involve, e.g.,specifying (programming) a time window or time threshold within thecardiac cycle that defines a time boundary for distinguishing conductedR-waves from nonconducted R-waves. If an R-wave occurs within the timewindow or after the time threshold, it is presumed to be a conductedR-wave. If it does not, it is presumed to be a nonconducted R-wave. Suchapproach is similar to that described above (relative to measuring thenatural conduction time and thereafter using such measured naturalconduction time as a reference against which subsequent AR intervals canbe compared) except that a measurement--at least a measurement made bythe pacemaker circuitry--of the AR interval is not necessarily required.Rather, a physician or other medical personnel (or even the pacemakermanufacturer) need only estimate (e.g., from prior experience, or fromexamining an EKG of the patient or from a sample of patients) what anappropriate "time window" might be for the patient, and then program orset such value into the pacemaker circuits.

Yet another technique for verifying conducted R-waves from nonconductedR-waves that may be used by the present invention is to require that atleast a prescribed number of R-waves, e.g., at least two, occur inconsecutive cardiac cycles at approximately the same time within thecardiac cycle, and more particularly within the AV interval of thecardiac cycle. If the prescribed number of R-waves repeat atapproximately the same time within the AV interval of consecutivecardiac cycles, that is a usually a good indication that the R-waves areconducted R-waves. This is because nonconducted R-waves (at least thosethat occur after an atrial event within the time window normally set foran AV interval), tend to be more erratic and less repetitive than doconducted R-waves. Thus, if a nonconducted R-wave occurs during the AVinterval, the chances are that it will not repeat, particularly overtwo, three, or four or more consecutive cardiac cycles.

Yet another technique that may be used in some embodiments of theinvention to discern conducted R-waves from nonconducted R-wavesinvolves measuring the amplitude of the R-wave. Ectopic and pathologicR-waves tend to be smaller in amplitude than do conducted R-waves.Hence, if the amplitude of the sensed R-wave exceeds a prescribedthreshold, then that provides some indication that the sensed R-wave isin fact a conducted R-wave.

A related technique that may be used in other embodiments of theinvention to discern conducted R-waves from nonconducted R-waves relatesto examining the morphology (shape) of the sensed R-wave and comparingsuch morphology to the morphology of known conducted R-waves. Thistechnique thus relies on the fact that conducted R-waves arecharacterized by distinct, recognizable, features, (e.g., in terms oftheir slope, amplitude, width, etc.) from nonconducted R-waves. Thus, bydetecting such characteristics, or by capturing the entire "image" ofthe sensed R-wave, and by comparing such captured information tocorresponding information previously obtained or defined from knownconducted R-waves, it is possible to classify the sensed R-wave as aconducted R-wave or a nonconducted R-wave. Circuitry for examining themorphology of an R-wave is disclosed, e.g., in U.S. patent applicationSer. No. 08/310,688, filed Sept. 22, 1994, which application isincorporated herein by reference.

In FIG. 6, a timing/event diagram is shown (similar to those of FIGS. 3and 4) that depicts an alternative technique for detecting a prematureventricular contraction (PVC) during a time when the short AVI has beeninvoked. As represented in FIG. 6, first and second A-pulses, generatedby the electronic pacemaker, each initiate the short AVI. At the end ofeach short AVI, the electronic pacemaker generates a V-pulse. Followinga third A-pulse 134, an R-wave 136 is detected. Because the R-wave 136is detected during the short AVI, however, as opposed to being detectedduring the original (long) AVI, such R-wave is assumed to be caused by apremature ventricular contraction (PVC), i.e., a late cycle ventricularectopic beat or a junctional beat, and not a conducted R-wave inresponse to an atrial event, either a naturally occurring P-wave or anA-pulse. The electronic pacemaker, having determined that the detectedR-wave 136 is due to a PVC, suppresses generation of a V-pulse followingthe short AVI, but does not restore the original (long) AVI. That is,the electronic pacemaker does not cease the ventricular supportresponse. Instead, the electronic pacemaker continues to carry out theventricular support response, including initiating the short AVIfollowing a fourth A-pulse 138 (and subsequent A-pulses in theabove-described series of A-pulses), and pacing the ventricle (bygenerating a V-pulse) at the end of such short AVI.

In reference to FIGS. 3 and 6, in another variation of the invention,the electronic pacemaker 10 detects a conducted R-wave during a "scan"cycle (i.e., a cycle using a longer AVI) only if an R-wave is detected(1) after an A-pulse, (2) before the longer AVI has expired, and (3)after the short AVI has expired. Note, in order to perform the functionsof this variation of the invention, both the original (long) AVI and theshort AVI must be initiated in response to the A-pulse of the scan cycleand allowed to time out in parallel, thereby defining a time windowcovering the time interval between the end of the short AVI and the endof the long AVI. (Other techniques may also be used to define anequivalent time window.) Any R-wave detected after the A-pulse of thescan cycle and before the longer AVI expires, but also before the shortAVI expires, is assumed to be a PVC. In response to such PVC, theelectronic pacemaker 10 suppresses generation of the V-pulse, butcontinues the ventricular support response, including using the shortAVI, following subsequent A-pulses. This variation of the invention notonly provides a precise criteria for determining what is and what is nota conducted R-wave, but also assures that the ventricular supportresponse is not terminated in response to a PVC, should a PVC occurafter an A-pulse during a "scan" cycle.

In FIG. 7, a timing/event diagram is shown that illustrates thecardiac/pacing events that might occur in response to an increasedpacing rate initiated by a rate-responsive electronic pacemaker. Asrepresented in FIG. 7, an A--A interval (i.e., the interval of timebetween A-pulses) decreases with each successive A-pulse, while the ARinterval remains about the same. Such a condition could easily occurwhen the electronic pacemaker 10 detects a need for an increased pacingrate. Problematically, however, such shortening of the A--A intervalwithout a concomitant shortening of the A-R interval could cause anA-pulse to be generated simultaneously with the previous R-wave. Thisphenomenon, known as AAI(R) pacemaker syndrome, results in the atriumbeing stimulated to contract against a closed AV valve (closed by thenaturally contracting ventricle). Such action results in very poorhemodynamic efficiency. Advantageously, the present invention preventsthe occurrence of AAI(R) pacemaker syndrome as described below.

Also shown in FIG. 7 are the original (long) AVI and the short AVI,which are shown for illustration purposes as being initiated in responseto each of the A-pulses. The first and second A--A intervals T₁, T₂ havea duration of about 1 second, and within the first and second A--Aintervals the original (long) AVI has a duration of about 300 ms, andthe short AVI has a duration of about 200 ms. The third A--A interval T₃has a duration of about 667 ms, and within the third A--A interval theoriginal (long) AVI has a duration of about 275 ms, and the shortatrioventricular interval has a duration of about 175 ms. The fourthA--A interval T₄ has a duration of about 546 ms, and within the fourthA--A interval the original (long) AVI has a duration of about 250 ms,and the short AVI has a duration of about 150 ms. Finally, the fifthA--A interval T₅ has a duration of about 462 ms, and within the fifthA--A interval the original (long) AVI has a duration of about 225 ms,and the short AVI has a duration of about 125 ms. Thus, for the exampleshown, the original (long) AVI and the short AVI are automaticallyshortened as the A--A interval is shortened by the rate-responsiveelectronic pacemaker. As a result of this shortening, the original andshort AVI's will not run over into the next cardiac cycle. As a result,the AVI's will expire some time before the generation of the nextA-pulse. Expiration of the AVI causes the generation of a V-pulse, whichin turn, invokes a ventricular depolarization/contraction, which in turnprevents any subsequent conducted ventricular contraction at the sametime as the subsequent generation of an A-pulse.

Advantageously, the changing of the original (long) and short AVI's(referred to herein as rate responsive modulation) assures that, whenthe A--A interval is shortened through rate-responsive modulation of theelectronic pacemaker, the AR interval is also shortened either naturallyor through the generation of V-pulses that invoke ventricularcontractions. As a result, the possibility of AAI(R) pacemaker syndromeis eliminated.

As seen in FIG. 7, the ventricular support response is still initiatedafter the fourth A-pulse, even though the original (long) and shortAVI's are being shortened. Note further that the "scan" operationdescribed above is preferably suppressed during the rate responsivemodulation when the detected cardiac rate exceeds a prescribedthreshold, e.g., 90 bpm (beats per minute). This allows the ventricularsupport response (short AVI) described herein to be initiated, but notterminated, whenever the detected cardiac rate exceeds the prescribedthreshold, i.e., whenever the heart rate is elevated. As the heart ratedeclines below the prescribed threshold, then the "scan" operation mayagain be invoked to determine if it is appropriate to lengthen the AVIback to its original (long) value.

In FIG. 8, an exemplary timing/event diagram is shown that illustratesone way in which an electronic pacemaker "learns" an average AR interval(ARI) of a patient. As illustrated in FIG. 8, the AR interval ismeasured over a prescribed number of cardiac cycles, e.g., 4-64. Aftersuch measurements, or during such measurements, the electronic pacemakeraverages the measured AR intervals to determine an average AR interval(AVG ARI). The average AR interval may be updated by the electronicpacemaker 10 during subsequent cardiac cycles during which the ARinterval is measured. For the five AR intervals illustrated in FIG. 8,the average AR interval is thus:

AVI_(AVE) =(175+180+170+172+180)/5=175.40 ms.

It should be apparent from the natural conduction times shown in FIG. 8that, while 175 represents the average AR interval, or average naturalconduction time, some R-waves occur before and after this value. Thus,in one embodiment, a prescribed safety margin (e.g., 25 ms) is includedwith the average AR interval so that all naturally conducted beats arepermitted to occur. In this embodiment, any R-wave which occurs duringthe short AVI and before the expiration of a reference interval(corresponding to the average R-wave minus 25 ms) is considered anonconducted beat. In an alternative embodiment, illustrated in FIG. 9,the reference interval is thought of as "natural conduction window"which includes a positive and a negative safety margin about the averageAR interval. Consequently, a sensed R-wave which occurs outside of aprescribed "natural conduction window," is considered a nonconductedbeat.

Besides computing a simple average, as described above, in order to"learn" the natural conduction time, it is noted that other techniquesmay also be used to learn the AR interval. Such other techniquesinclude, e.g., computing a statistical "mean," "median," "mode," etc. ofthe measured AR intervals, performing a least-squared analysis of themeasured AR intervals, sampling the measured AR intervals, and the like.

As a further option, the AV interval can be "learned," as describedabove, and then reported to a physician through, e.g., the externalprogrammer 48. The physician may then be given the option to program thepacemaker to use the reported "learned" AV interval, or the option tooverride the reported "learned" AV interval and substitute another valuein its place. In this way, the physician is afforded the opportunity toeither use the reported "learned" AV interval or select another AVinterval.

Once the AR interval has been "learned," the "learned" value maythereafter be used as a "standard" or "reference" against whichsubsequent AR intervals may be compared. This process is illustrated inFIG. 9.

In FIG. 9, any R-wave is considered to be a nonconducted beat (e.g., apremature ventricular contraction (PVC)) if it is detected after anA-pulse and outside of a natural conduction window defined as the"learned" AR interval plus or minus a prescribed safety factor. (Notethat the safety margin may be a fixed value, e.g., 25 ms, or aprogrammable value of from, e.g., 0 to 100 ms.) As described previously,when a PVC is detected, it is ignored by the electronic pacemaker 10 forpurposes of determining whether the ventricular support response isstill needed. The electronic pacemaker does, however, suppressgeneration of the V-pulse within the cardiac cycle in which the PVCoccurs.

As seen in FIG. 9, when the ventricular support response is initiated,the short AVI follows a first A-pulse 156, which short AVI is followedby a first V-pulse 158. The first V-pulse 158 is generated in responseto the expiration of the short AVI without an R-wave having beendetected. In response to the first V-pulse 158, a first R-wave 159 isevoked. Following the first R-wave 159, a second A-pulse 160 isgenerated, which is followed by a second R-wave 162. The second R-wave162 is detected outside of the natural conduction window (NCW), yetbefore the end of the short AVI. Thus, the second R-wave 162 is presumedto be a nonconducted beat, and is therefore ignored by the electronicpacemaker for purposes of determining whether the ventricular supportresponse is still needed. The nonconducted R-wave 162 does, however,inhibit the generation of a V-pulse at the conclusion of the AVI thatfollows the second A-pulse 160.

In FIG. 9, a third A-pulse 164 is shown following the second R-wave 162.No R-wave is detected before the expiration of the short AVI thatfollows the A-pulse 164. Following the second V-pulse 166 a thirdR-wave, invoked by the V-pulse 166, is shown. A fourth A-pulse 168follows. The fourth A-pulse 168 again initiates the short AVI, as wellas the natural conduction window. Following the fourth A-pulse 168, butbefore the expiration of the short AVI, a fourth R-wave 170 is detected.Because the R-wave 170 is detected within the natural conduction window,and before the short AVI expires, it is not considered a PVC, but israther considered a naturally conducted R-wave, and therefore signalsthat the ventricular support response is no longer needed. Note in thisinstance that the indication that ventricular support is no longerneeded occurs during a cardiac cycle that is not a scan cycle. Becausethe ventricular support response is no longer needed, the original(long) AVI is initiated in response to a fifth A-pulse 172 that followsthe R-wave 170, along with the natural conduction window. A fifth R-wave174 follows the A-pulse 172 within the natural conduction window, andbefore the end of the original (long) AVI. The occurrence of the R-wave174 within this window indicates that the ventricular support responseremains unneeded.

As described above, it is seen that the invention is able to detectwhether there is a continued need for the ventricular support response,while accurately distinguishing between PVC'S, which do not signal theneed to end the ventricular support response, and conducted R-waves,which do signal that the ventricular support response may be ended.

It is noted that thus far, the invention has been described primarily interms of atrial-based pacing. As is known in the art, dual-chamberpacing may be either atrial based or ventricular based. In atrial-basedpacing, the occurrence of an atrial event starts the appropriate timersthat define the duration of the basic cardiac cycle, or A--A interval(which A--A interval includes the time interval from an A-pulse to anA-pulse, or from a P-wave to an A-pulse). The A/P-A interval includesthe AV or PV interval and an atrial escape interval, AEI, both of whichare keyed off of, or start, upon the occurrence of an atrial event,i.e., either the sensing of a P-wave or the generation of an A-pulse. Inventricular-based pacing, in contrast, the basic pacing cycle, or A--Ainterval, includes the AV interval followed by a VA interval. The AVinterval begins upon the occurrence of an atrial event, either a P-waveor an A-pulse. The VA interval begins upon the occurrence of aventricular event, either an R-wave before the timing out of the AVinterval, or the generation of a V-pulse upon the timing out of the AVinterval. Thus, when an R-wave occurs, the VA interval begins sooner inthe cardiac cycle than it would have had the R-wave not occurred, andthe basic A--A interval that defines the basic cardiac cycle is madeshorter.

In some instances, where ventricular-based pacing is employed, it isdesirable to prevent the A--A interval from being made shorter. Such isaccomplished by adding to the VA interval the same amount of time bywhich the AV interval is shortened when the AVI is switched from a longAVI to a short AVI. That is, if the AVI is shortened by an amount D,then the VA interval is lengthened by the same amount D so as tomaintain the same basic time for the A--A interval. Similarly, whenoperating in a ventricular-based pacing mode, and if the AVI islengthened by an amount D, then the VA interval is shortened by the sameamount D so as to maintain the same basic time for the A--A interval.

When operating in an atrial-based pacing mode, such as the functionalAAI pacing mode, there is no need to adjust any timing intervals becauseboth the AV interval and AEI are keyed off of an atrial event and timer.

Turning next to FIGS. 13 and 14, a specific example is shown of thepresent invention as applied to atrial based timing (FIG. 13) andventricular based timing (FIG. 14). In both examples, it is assumed thatthe basic rate of the pacemaker has been set to 60 ppm (pulses perminute), corresponding to a basic pacing rate interval of 1000 msec. Itis further assumed that the programmed value of AVI has been set to 150msec, and that the hysteresis delta (sometimes referred to as thehysteresis interval) has been set to a +100 msec. Note that with thehysteresis delta set to a positive value, that such positive value isadded to the programmed AVI in order to define the long AVI used by thepresent invention when ventricular support is not anticipated as beingneeded, i.e., when conductive R-waves are anticipated to be present.Thus, the long AVI interval used in the example shown in FIGS. 13 and 14is 250 msec, while the short AVI interval is 150 msec.

The basic hysteresis technique illustrated in the examples shown inFIGS. 13 and 14 is that if conduction is detected during the programmedAVI (i.e., if a conducted R-wave is sensed that signals the end of an ARor PR interval), then a new AVI, AVI_(N), is established that is equalto the programmed AVI plus the hysteresis delta, or AVI_(N)=AVI+Hysteresis Delta=150+100=250 msec. When loss of conduction isdetected, (i.e., if no R-wave occurs during AVI_(N), thereby forcing thepacemaker circuits to issue a V-pulse at the conclusion of AVI_(N)),that is, upon the first PV or AV interval when using AVI_(N), thenAVI_(N) is canceled, and AV interval returns to its programmed value,AVI. Thus, the AV interval is set to its new (long) value whenconduction is detected, and returns to is programmed (short) value whenloss of conduction is present. Should 255 consecutive PV or AV intervalsoccur without conduction being sensed, then a long AVI is established inorder to scan for, or unmask, the presence of intrinsic conduction.

This unmasking process is illustrated in FIG. 13. That is, the V-pulse302 in FIG. 13 represents the end of the 254th AV interval without anR-wave having been sensed. The V-pulse 304 thus represents the end ofthe 255th AV interval without an R-wave having been sensed. Accordingly,in accordance with the basic unmasking or scanning technique employed bythe example of the invention shown in FIG. 13, the AV interval of the256th cycle, which cycle begins with A-pulse 306, is extended to 250msec. Such extension uncovers R-wave 308. Once R-wave 308 is confirmedas a conducted R-wave, which confirmation may be made using any of thetechniques previously described (such as requiring that an R-wave 308 becontinuously present over at least two consecutive cycles), then the new(long) AV interval, AVI_(N), takes effect. The long AVI_(N) remains ineffect for so long as continuous AR or PR intervals are present, i.e.,for so long as R-wave continue to be sensed. As soon as an R-wave failsto be sensed, as evidenced e.g., by the timing out of the AV/PV intervaland the generation of a V-pulse 310, then the next AV interval for usein the next cardiac cycle is shortened to its programmed (short) valueof 150 msec. Such programmed value of AVI is used, in accordance withthe example shown in FIG. 13, until conduction returns, or if conductiondoes not return, for at least another 255 cardiac cycles.

Note for the atrial-based timing example shown in FIG. 13, that theA-to-A interval remains constant at 1000 msec. This is because the basicpacemaker timing is keyed off of the occurrence of an atrial event,either a P-wave or an A-pulse. Note also, as previously described, thata programmed PV interval (PVI) may be somewhat shorter than theprogrammed AVI. For example, if the AVI is programmed to 150 msec., thenthe PVI might be programmed to 125 msec. Such different in programmedPVI from the programmed AVI carries over to the hysteresis values. Thatis, with a positive hysteresis delta of 100 msec., the long AVI_(N) is150 msec., and the long PV interval, PVI_(N), would be 125+100=225 msec.

The positive hysteresis example illustrated in FIG. 14 is the same asthe example shown in FIG. 13, except that ventricular-based timing isused. That is, a V-pulse 320 in FIG. 14 represents the end of the 254thAV interval without an R-wave having been sensed. In ventricular basedtiming, note that the AV interval is cascaded in series with a VAinterval in order to define a basic pacing rate. Thus, as shown in FIG.14, if the programmed AV interval is set to 150 msec., and if the basicpacing interval is 1000 msec., then the VA interval must be set to 850msec., so that the sum of the AV interval and the VA interval will equal1000 msec. Continuing with the example shown in FIG. 14, the V-pulse 322represents the end of the 255th AV interval without an R-wave havingbeen sensed. Accordingly, in accordance with the basic unmaskingtechnique employed by the example shown in FIG. 14, the AV interval ofthe 256th cycle, which AV interval begins after the timing out of thepreceding VA interval, begins with A-pulse 324 and is extended to 250msec. Such extension uncovers R-wave 326, which (for purposes of theexample shown in FIG. 14) occurs at about 170 msec. into the AVinterval. Because ventricular based timing is used, R-wave 326 alsodefines the starting point for the next VA interval. Thus, it is seenthat the 256th cardiac cycle has a duration of 170 msec+850 msec=1020msec. Once R-wave 326 is confirmed as a conducted R-wave, whichconfirmation may be made using any of the techniques previouslydescribed (such as requiring that a similar R-wave 328 also be presentin the next cardiac cycle), then the new (long) AV interval, AVI_(N),takes effect. The long AVI_(N) remains in effect for so long ascontinuous AR or PR intervals are present, i.e., for so long as R-wavescontinue to be sensed. As soon as an R-wave fails to be sensed, asevidenced e.g., by the timing out of the AV interval and the generationof a V-pulse 330, then the next AV interval for use in the next cardiaccycle is shortened to its programmed (short) value of 150 msec. Suchprogrammed value of AVI is used, in accordance with the example shown inFIG. 14, until conduction returns, or if conduction does not return, forat least another 255 cardiac cycles.

Note for the ventricular-based timing example of FIG. 14, that theA-to-A interval does not remain constant. So long as V-pacing isprovided using the programmed AVI, the A-to-A interval remains at 1000msec. However, when the new AV interval, AVI_(N), is used, the durationof a given cardiac cycle varies as a function of the time within theAVI_(N) when an R-wave is sensed. In the example of FIG. 14, when anR-wave occurred at about 170 msec into the AVI_(N), the A-to-A intervalwas extended about 20 msec., or to 1020 msec. When an R-wave did notoccur during AVI_(N), causing V-pulse 330 to be generated upon thetiming out of the AVI_(N), then the next A-to-A interval was extended afull 100 msec (the duration of the hysteresis delta) to 1100 msec.

As described above, it is thus seen that the present invention providesan implantable pacemaker that automatically sets its AV interval toeither a long or short value in order to optimize the hemodynamicperformance of the patient's heart with which the pacemaker is used.More particularly, it is seen that the invention automatically invokesan AV shortening procedure that shortens the AV interval, e.g., wheneverAV block occurs, and that thereafter automatically searches for thereturn of AV conduction, e.g., in accordance with a prescribed schedule,so that when AV conduction returns the AV interval may be reset to itsoriginal (longer) value.

What is claimed is:
 1. A method of controlling the operation of adual-chamber electronic pacemaker, the pacemaker having pulse generatingmeans for selectively generating an atrial stimulation pulse (A-pulse)and a ventricular stimulation pulse (V-pulse); timing means forgenerating specified time intervals, including an AV interval; sensingmeans for sensing selected cardiac events, including P-waves andR-waves; means for controlling the pulse generating means as a functionof whether the sensing means senses certain prescribed events during agiven time interval generated by the timing means, the methodcomprising:(a) measuring an AR interval defined as a time intervalbetween an atrial event and a sensed R-wave, the atrial event comprisingeither the generation of an A-pulse or the sensing of a P-wave; (b)averaging the AR interval over a prescribed number of cardiac cycles todefine a "learned" AR interval, the "learned" AR interval correspondingto a natural conduction time of the heart; (c) determining whether anR-wave is naturally conducted by comparing the AR interval of an R-wavewhich occurs during an original AV interval with the "learned" ARinterval, the original AV interval comprising a time interval generatedby the timing means that commences with an atrial event and terminates aprescribed period of time thereafter; (d) shortening the AV interval bya specified amount to produce a short AV interval whenever AV conductionis determined not to be present; and (e) returning the AV interval tothe original AV interval whenever AV conduction has returned, andkeeping the AV interval at its short value whenever AV conduction hasnot returned.
 2. The method of claim 1, further comprising regularlychecking, whenever the short AV interval is being used, to see if AVconduction has returned.
 3. The method of claim 2, wherein the regularlychecking step comprises scanning for an occurrence of an R-wave on aregular basis by:(1) lengthening the short AV interval back to theoriginal AV interval; (2) sensing whether an R-wave occurs before thetermination of the AV interval lengthened in step (1), and if someasuring an AR interval associated with such sensed R-wave; and (3)comparing the measured AR interval of the R-wave sensed in the sensingstep (2) with the "learned" AR interval to determine if such R-waverepresents a return of AV conduction.
 4. The method of claim 3,wherein:step (1) comprises incrementally lengthening the AV intervalback to the original AV interval in small increments over severalcardiac cycles; and step (2) comprises sensing whether an R-wave occursbefore the termination of the incrementally lengthened AV interval ofeach cardiac cycle.
 5. The method of claim 2, wherein the regularlychecking step comprises scanning for an occurrence of an R-wave on aregular basis by:(1) lengthening the short AV interval back to theoriginal AV interval; (2) starting a reference time interval upon anoccurrence of an atrial event during each cardiac cycle, the referencetime interval comprising an interval within a prescribed safety marginof the "learned" AR interval; and (3) sensing whether an R-wave occursafter the termination of the reference time interval but before thetermination of the original AV interval, and if so making a presumptionthat AV conduction has returned.
 6. The method of claim 5, wherein step(2) comprises setting the reference time interval equal to the "learned"AR interval in addition to a prescribed safety margin.
 7. The method ofclaim 6, wherein the prescribed safety margin comprises at least 25 ms.8. The method of claim 5, wherein:step (1) comprises incrementallylengthening the AV interval back to its original AV interval in smallincrements over several cardiac cycles; step (2) comprises starting thereference time interval during each cardiac cycle; and step (3)comprises sensing whether an R-wave occurs after the termination of thereference time interval but before the termination of the incrementallylengthened AV interval during each cardiac cycle, and if so making apresumption that AV conduction has returned, incrementally lengthened AVinterval of each cardiac cycle.
 9. The method of claim 8, wherein step(2) comprises making the reference time interval be equal to the"learned" AR interval within a prescribed safety margin.
 10. The methodof claim 7, wherein the prescribed safety margin comprises at least 25ms.
 11. The method of claim 1, wherein the regularly checking stepcomprises scanning for the occurrence of an R-wave every n cardiaccycles, where n comprises an integer of between 4 to 16 cardiac cycles.12. The method of claim 11, further comprising increasing n to aninteger of between 127 and 256 whenever an R-wave does not occur after afirst scanning therefor.
 13. The method of claim 11, wherein step (c)comprises detecting whether an R-wave occurs in a cardiac cycle (1)after an atrial event, and (2) before the original AV interval hasterminated, and (3) after the short AV interval has terminated.
 14. Themethod of claim 13, further comprising resuming that any R-wave detectedafter the atrial event and before the short AV interval has terminatedis representative of a nonconducted R-wave.
 15. The method of claim 1,wherein the electronic pacemaker further includes physiological sensingmeans for sensing a physiologic condition of a patient; and means fordefining a sensor-indicated rate (SIR), the SIR providing a measure of aminimum acceptable cardiac rate for a given sensed physiologic conditionof the patient, the method further comprising:adjusting the amount bywhich the original AV interval is shortened to produce the short AVinterval as a function of the SIR.
 16. The method of claim 15, furthercomprising adjusting the original AV interval as a function of the SIR.17. A dual-chamber electronic pacemaker, comprising:pulse generatingmeans for selectively generating an atrial stimulation pulse (A-pulse)and a ventricular stimulation pulse (V-pulse); timing means forgenerating specified time intervals, including an AV interval; sensingmeans for sensing selected cardiac events, including P-waves andR-waves; control means for controlling the pulse generating means as afunction of whether the sensing means senses certain prescribed eventsduring a given time interval generated by the timing means, the controlmeans including:means for measuring an AR interval defined as a timeinterval between an atrial event and a sensed R-wave, the atrial eventcomprising either a stimulation A-pulse or a sensed P-wave; means foraveraging the AR interval over a prescribed number of cardiac cycles todefine a "learned" AR interval, the "learned" AR interval correspondingto a natural conduction time of the heart; means for determining whethera sensed R-wave is a conducted R-wave by comparing the AR interval of anR-wave which occurs during an original AV interval with the "learned" ARinterval, the original AV interval comprising a time interval generatedby the timing means that commences with an atrial event and terminates aprescribed period of time thereafter; means for shortening the AVinterval by a specified amount to produce a short AV interval wheneverAV conduction is determined not to be present; and means for returningthe AV interval to a long AV interval whenever AV conduction hasreturned, and for keeping the AV interval at its short value whenever AVconduction has not returned.
 18. The pacemaker of claim 17, furthercomprising means for regularly checking, whenever the short AV intervalis being used, to see if AV conduction has returned.
 19. The pacemakerof claim 17, wherein the means for regularly checking to see if AVconduction has returned comprises scanning means for scanning for anoccurrence of an R-wave, the scanning means comprising:means forlengthening the short AV interval back to the original AV interval;means for sensing whether an R-wave occurs before the termination of thelengthened AV interval, and if so, for measuring an AR intervalassociated with such sensed R-wave; and means for comparing the measuredAR interval associated with the sensed R-wave against the "learned" ARinterval to determine if such R-wave represents a return of AVconduction.
 20. The pacemaker of claim 19, wherein:the scanning meanscomprises means for incrementally lengthening the AV interval back toits original AV interval in small increments over several cardiaccycles; and the means for sensing whether an R-wave occurs comprisesmeans for sensing whether an R-wave occurs before the termination of theincrementally lengthened AV interval of each cardiac cycle.